1) Technical Field
The present invention relates to a method to determine the injection pattern in the compression stroke of the combustion cycle of the cylinders of a direct-injection internal combustion engine.
2) Brief Description of the Relevant Art
A spark-ignited, direct-injection internal combustion engine typically includes a number of cylinders, each of which is provided with a respective piston, which cyclically slides within the cylinder and is mechanically connected using a connecting rod to a crankshaft to transmit to the drive shaft itself the force generated by the combustion within the cylinder, with a respective injector partially arranged inside the cylinder and with a respective spark plug, which is cyclically controlled by an electronic control unit to produce a spark between its electrodes and thus determine the ignition of the compressed gases in the cylinder itself. The internal combustion engine further includes an intake manifold, which is connected to each cylinder by means of one or more intake valves, and an exhaust manifold, which is connected to each cylinder by means of one or more exhaust valves and which leads to an emission pipe to emit the gases produced by the combustion into the atmosphere.
The four-stroke combustion cycle of each cylinder consists of four strokes in sequence: an intake stroke, a compression stroke, an expansion stroke and an exhaust stroke. At the end of each combustion cycle, two complete revolutions will have been completed by the drive shaft, thus covering a rotation equal to 720°. The piston is initially at top dead center and, during the intake stroke, descends to bottom dead center by generating a vacuum in the cylinder which, as the respective intake valve opens, recalls air from the intake pipe covering half a revolution of the drive shaft and exploring an angle from 0° to 180°. Fuel is injected in the combustion chamber during the intake stroke and/or during the subsequent compression stroke. During the subsequent compression stroke, the piston rises from bottom dead center to top dead center, thus compressing the air and fuel mixture which is in the cylinder, causing an increase of pressure and of temperature, covering half a revolution of the drive shaft and exploring an angle from 180° to 360°.
During the expansion stroke, the electrodes of the spark plug produce the spark which ignites the air and fuel mixture in the cylinder starting the combustion itself, which produces an increase of temperature and pressure in addition to being the only step of the combustion cycle in which useful work is generated. The piston is pushed from top dead center to bottom dead center by covering half a revolution of the drive shaft and exploring an angle from 360° to 540°. Finally, in the exhaust stroke, the piston is at bottom dead center again and moves up to top dead center to eject the burnt gases, which are introduced into the exhaust pipe, by means of the respective exhaust valve, covering half a revolution of the drive shaft and exploring an angle from 540° to 720°.
The overall quantity of fuel to be injected for each four-stroke combustion cycle can be determined as a function of the torque to be delivered to the drive wheels which determines the quantity of air that the engine must aspirate. The overall quantity of fuel to be injected for each four-stroke combustion cycle of each cylinder may be injected, alternatively, either during the intake stroke, or during the compression stroke, or even during both the intake stroke and the compression stroke.
The definition of the injection pattern (i.e. the determination of the stroke or strokes of the combustion cycle during which the quantity of fuel is either completely or partially injected) is extremely critical to be able guarantee a correct mixing of air and fuel and to prevent the fuel from fouling the crown of the piston or the walls of the cylinder and causing the formation of particulate.